The present invention relates to improvements in loudspeaker systems designed to reproduce relatively low or bass frequencies.
In the prior art of constructing loudspeaker systems designed for the reproduction of relatively low frequencies various systems have evolved broadly classifiable into large systems and small systems. In large systems the loudspeaker dimensions are comparable to at least a substantial fraction of the wavelength of sound in air of the lowest frequency to be reproduced satisfactorily. This group includes horn loaded and other large baffle designs.
The other group of small loudspeaker systems contains all those designs in which the loudspeaker system dimensions are a relatively small fraction of the lowest sound wavelength to be reproduced in air. This group includes the various direct radiator loudspeaker systems in which the loudspeaker acts as a vibrating piston mounted in an enclosure so that the sound radiated by both sides of that piston surface are effectively controlled. The front or outside surface radiates the desired acoustic waves directly while the rear or inside surface radiates sound into the enclosure, thereby preventing direct simultaneous radiation of sound of opposite phase which would tend to cancel acoustic output. It is recognized that numerous methods have been devised to utilize the radiation from the rear to enhance loudspeaker output at certain low frequencies.
Closed box or "infinite baffle" types of loudspeakers employing a moving coil dynamic loudspeaker driver when reproducing electrical signals may be characterized by the general equation: ##EQU1## where W is the radiated sound power, Wo is the reference sound power related to equivalent input power and loudspeaker efficiency; S is frequency relative to the resonance frequency of the loudspeaker in the enclosure; and Q is the quality factor indicating the reciprocal of the damping factor of the loudspeaker. The equation may be recognized as that of a high-pass filter of the second order which would be maximally flat (no peak in response) if Q were equal to 1/.sqroot.2. Assuming that frictional losses can be neglected and assuming that the loudspeaker suspension stiffness is small compared to the stiffness of the air-spring formed by the volume of air contained in the enclosure, the following factor holds for a free-standing loudspeaker: ##EQU2## where fc is the resonant frequency between radiator mass and the enclosure volume V (measured in m.sup.3), Q is the quality factor and .eta. is the relatively efficiency expressed as a fraction of acoustic output power to electrical input power, expressed as the square of input voltage divided by voice coil resistance.
Numerous attempts have been made to extend the low frequency range by an array of arrangements. For example, the use of a tuned vent or port permits the low frequency range to be extended with the overall response following the form of a fourth order filter. If such a filter were to be maximally flat, the low frequency -3 dB limit would be 0.841 times the -3 dB limit of a second order loudspeaker with the same moving mass and cabinet size. If the same efficiency were maintained along with the same cabinet size, the -3 dB limit would be 0.726 times the -3 dB limit of the second order loudspeaker.
The disadvantage of such an arrangement is the requirement of a substantially increased motor strength requiring larger magnets. Further disadvantages include the production of undesired rushing noises of relatively high velocity air passing through the vent opening and the existence of higher frequency resonances of the interior of the enclosure, causing substantial coloration or aberration of the frequency response of the loudspeaker.
In the present invention, the disadvantages of ported or vented loudspeakers have been overcome by use of a closed box loudspeaker system while the extended low frequency response has been obtained by the insertion of a passive electrical network in the loudspeaker connections to the output terminals of the driving amplifier. As an unexpected benefit of this invention certain critical relationships were discovered between loudspeaker and cabinet parameters on one hand and electrical network values on the other hand. Further relationships were discovered which permit the design of loudspeaker systems to follow the requirements of transfer functions of higher order than has been thought to be possible up to now.